The basic processing scheme of a personal computer does not directly facilitate display of visual images showing continuous motion, as particularly exemplified by standard television transmissions. Processing and displaying pictures at this real time rate involves handling a relatively large data stream.
It can readily be seen there are numerous potential uses for the capability of viewing a motion picture or TV type image on an inset portion of a PC screen. In addition to merely watching television, surveillance and security systems come to mind--comparing a closed-circuit view of a person with file copies of photographs and clearance or authorization, for example. Viewing of testing operations (e.g.: wind tunnels or displays of changing data such as oscillographs monitored by remote cameras) while reviewing or correcting calculated data is another possibility. Reviewing forensic simulations on magnetic tape recordings while preparing or reviewing commentary or testimony is another, as well as comparison of still file photographs therewith. Other applications are not difficult to conceive.
Visual images for display on a PC are made up of pixels, or discrete image elements, each pixel requiring an eight bit word for definition. It can be seen that the operating speed of a PC will determine whether or not it is possible to achieve motion display with the smoothness which is characteristic of the standard thirty frame per second real time rate. When the PC lacks sufficient speed to display real time motion, edited or intermittent motion display could still be very useful in many applications.
Prior art methods transmit only one pixel of video at a time on the computer's internal bus, resulting in a transmission time of as much as one second or more for a black and white image, considerably longer for color, which is clearly inadequate for smooth display of images involving motion.
Prior art devices for achieving display of moving images on a PC screen from analog sources have customarily provided extra processing capability to bypass the internal computer bus cycle processing or reduce the machine time required for the picture display. This results in an increase in equipment expense, as processors are relatively expensive, as well as increasing the bulk and installation complexity of the equipment.
Applicant is aware of two current art products which seek to achieve a result similar to the current invention.
1. A device called the MassMicro, selling for about $3,000, it is believed. This device requires two plug-in cards and includes its own video output card, producing a video overlay which it mixes directly with the picture displayed by the PC. The host PC's main processor never sees the data being processed by the MassMicro. PA1 2. A device called the OrangeMicro, selling for about $4,000, it is believed. This device also requires two plug-in cards, and has its own processor, taking over the PC data bus as a bus master processor, so again the PC main processor never sees the data being handled by this device. PA1 a: In what may be termed the real time (eight-bit color) mode, analog mux 41 directs A-D converter 42 to the eight-bit color mode. In this mode, the green component signal is transferred by analog mux 41 to A-D converter 42, while the red signal goes direct to A-D converter 43 and the blue signal to A-D converter 44. Data buffer 46 continuously accepts eight-bit color words (three red, three green, two blue) and transfers them for storage in video frame memory 47, from which complete video frames are transferred to the host computer NuBus, as described subsequently. PA1 b. In an alternative mode providing a slow rate non-real-time display of much higher resolution (and many more color shades), analog mux 41 and data buffer 45 are commanded to assemble 24-bit color words, each of all one color (21 color bits from three seven-bit words, plus three dummy bits). Mux 41 selects the proper color component and transfers it to A-D converter 42, which provides seven-bit color words to data buffer 45 for storage in memory 47. In this mode, an entire frame of red signals is assembled and transferred to the computer, then a frame of green, then of blue. Software provided with the invention for the Macintosh provides for display of a resultant high-resolution color frame at a slow repetition rate, in what might be considered a freeze frame mode. It might be noted that the dither circuit has very little effect in this color mode. PA1 a. When a complete TV picture has been assembled in the memory, the DigiVideo transmits a signal to the PC processor that a picture frame is ready. PA1 b. When the main PC program is ready to read the picture for display, it addresses the DigiVideo by a signal known in Macintosh language as "myslot", initiating data readout from that source in time intervals of 500 nanoseconds .vertline.ns.vertline. each. PA1 c. During the allotted 500ns interval, the MicroTv Receives a clock signal every 100ns from the Macintosh PC: On the "myslot" pulse, the controller in the invention's novel data transfer stage directs transfer of video data from the RAM to four output registers or buffers, the first three of which are uni-directional latched buffers, the fourth being bi-directional. On each of the four successive clock counts at 100 ns intervals, an eight-bit data word describing a pixel of information is transferred in parallel into one of the four eight-bit output registers, in order. On the fourth clock pulse in addition to loading the data word in the fourth buffer, the controller directs output in parallel of all four words to the PC which accepts them in correct order as one 32-bit word for Macintosh processing. The DigiVideo then acknowledges and signals ready again. PA1 d. The PC continues to address the DigiVideo under its program control, each sample time being 500 ns, during which time four pixels are transferred as described in 7 above, the sample rate being determined by program rate and relative priority, as described below. When the complete picture has been read, the MicroTv assembles another complete frame in its RAM, signals the PC that a complete picture is ready and the process repeats under control of the PC program.
In addition to their expense, their requirement for two expansion slots in the PC, and their interference with control by the host PC, neither of these devices is capable of the data delivery speed necessary for smooth motion display on the PC screen.
The current invention provides, on a single plug-in video card at a much lower cost than either of the above-mentioned devices, smooth video motion at up to thirty frames per second. The current invention not only provides, by its novel data transfer stage, picture data at a much higher rate than either of the above devices, but does so under the master control of the host PC, rather than overriding its processing, as do the two devices mentioned above.